=Marine Protozoa.=--One usually thinks of the ocean as the home of the whales and the seals and the sea-lions, and of the countless fishes, the cod, and the herring, and the mackerel. Those who have been on the seashore will recall the sea-urchins and starfishes and the sea-anemones which live in the tide-pools. On the beach there are the innumerable shells, too, each representing an animal which has lived in the ocean. But more abundant than all of these, and in one way more important than all, are the myriads of the marine Protozoa.
Although the water at the surface of the ocean appears clear and on superficial examination seems to contain no animals, yet in certain parts of the ocean (especially in the southern seas) a microscopical examination of this water shows it to be swarming with Protozoa. And not only is the water just at the surface inhabited by one-celled animals, but they can be found in all the water from the surface to a great depth below it. In a pint of this ocean-water there may be millions of these minute animals. In the oceans of the world the number of them is inconceivable. And it is necessary that these Protozoa exist in such great numbers, for they and the marine one-celled plants (Protophyta) supply directly or indirectly the food for all the other animals of the ocean.
Among all these ocean Protozoa none are more interesting than those belonging to the two orders Foraminifera (fig. 11) and Radiolaria. The many kinds belonging to these orders secrete a tiny shell (of lime in the Foraminifera, of silica in the Radiolaria) which encloses most of the one-celled body. These minute shells present a great variety of shape and pattern, many being of the most exquisite symmetry and beauty. The shells are perforated by many small holes through which project long, delicate, protoplasmic pseudopodia. These fine pseudopodia often interlace and fuse when they touch each other, thus forming a sort of protoplasmic network outside of the shell. In some cases there is a complete layer of protoplasm--part of the body protoplasm of the Protozoan--surrounding the cell externally.
[Illustration: FIG. 11.--_Rosalina varians_, a marine protozoan (Foraminifera) with calcareous shell. (After Schultze.)]